1. Field of the Invention
The present invention relates generally to positioning devices, and more particularly to an ultra-precision positioning assembly.
2. Discussion of Background Art
Positioning devices are widely employed for a variety of tasks in many fields. Depending upon the task, positioning devices typically have tolerances from a few millimeters to a few nanometers. Ultra-precision positioning devices, having tolerances in the nanometer range, are used in applications such as machining (single-point, fixed-abrasive, and loose abrasive), positioning (measurement and inspection devices, optical and laser devices, and astronomy), optics manufacturing and testing (glass, ceramic, and metal), and micro-fabrication (large-scale three-dimensional device) applications.
As tolerances become smaller and the mass of an object to be positioned increases, the positioning device""s expense tends to exponentially increase. For example, very large Potassium Di-hydrogen Phosphate (KDP) crystal plates (typically sized as 470 mmxc3x97560 mm workpieces ranging in thickness from 10 mm to 100 mm) need to be fabricated for the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. These massive plates must be fabricated on a machine capable of producing a 1.5 micron flat surface, with a depth-of-cut resolution of 50 nm, and able to produce wedge angles in two directions up to plus or minus 0.25 degree. Such a machine would need to possess a difficult-to-achieve combination of long vertical travel, fine positioning resolution, angular tip and tilt capability, and a high degree of mechanical stiffness.
Currently, no such positioning apparatus has been found to exist. Currently available ultra-precision positioning apparatus"" are highly complex, costly, and cannot achieve the required performance due to an inability to accommodate the combined requirements for long vertical travel, fine positioning resolution, angular tip and tilt capability, and high mechanical stiffness. In addition, currently available ultra-precision positioning apparatus, when configured for the example use, suffer an additional performance penalty due to overturning moments which tend to distort and disfigure a positioning apparatus during operation. An overturning moment is well known and generally defined as a torque which tends to rotate a body out of a reference plane.
In response to the concerns discussed above, what is needed is an ultra-precision positioning assembly that overcomes the problems of the prior art.
The present invention is an ultra-precision positioning assembly. Within the apparatus of the present invention, a slide base provides a foundational support. A slide plate moves with respect to the slide base along a first geometric axis. A ball-screw displaces the slide plate with respect to the slide base along the first geometric axis. A linking device directs a primary force vector into a center-line of the ball-screw. The linking device consists of a first link which directs a first portion of the primary force vector to an apex point, located along the center-line of the ball-screw, and a second link for directing a second portion of the primary force vector to the apex point. A set of rails, oriented substantially parallel to the center-line of the ball-screw, direct movement of the slide plate with respect to the slide base and are positioned such that the apex point falls within a geometric plane formed by the rails and the center-line of the ball-screw.
In other aspects of the invention, slide bearings, connecting the slide plate to the rails, constrain a portion of the primary force vector which tends to move the apex point away from the center-line of the ball-screw. A stepper motor rotates the ball-screw to displace the slide plate with respect to the slide base. A piezoelectric actuator alternatively pushes on the ball-screw to displace the slide plate with respect to the slide base.
In another embodiment of the invention, the slide base, the slide plate, the ball-screw, the linking device, rails, slide bearings, stepper motor, and piezoelectric actuator, together form a slide assembly, and multiple slide assemblies are distributed about a platform. In such a configuration, the platform may be raised and lowered, or tipped and tilted by jointly or independently displacing the slide plates.
The positioning assembly of the present invention is particularly advantageous over the prior art because in one embodiment, a 75 kg workpiece having a 470 mmxc3x97560 mm footprint can be positioned in a vertical work zone of 100 mm with a resolution of 50 nm, and tipped and tilted through angles up to plus or minus 0.25 degrees with a resolution of 0.5 microradian. The positioning assembly is also capable of achieving a low overall apparatus compliance of 40 nm/N (8 microinch/lb), and a structural first natural frequency above 100 Hz. A low overall compliance is analogous to a high degree of mechanical stiffness.
These and other aspects of the invention will be recognized by those skilled in the art upon review of the detailed description, drawings, and claims set forth below.